Carcinogenesis involves a series of somatic genetic changes affecting the structure and/or expression of oncogenes and tumor suppressor genes. Secondary genetic changes and epigenetic mechanisms may also be necessary to allow small nests of malignant cells to form clinically apparent primary and metastatic tumors. In the case of solid neoplasms, for example, it is well known that growth beyond diameters of 1-2 mm depends on formation of supporting stroma of newly formed blood vessels, usually accompanied by reactive stromal fibroblasts, lymphoid and phagocytic infiltrates, and extracellular matrix proteins. While cells of reactive tumor stroma are not transformed, they may differ from corresponding cells of normal tissues in proliferative activity, as well as in the expression of regulatory peptides, proteolytic enzymes, ECM proteins and cell surface antigens. Consequently these may provide additional targets for pharmacological and immunological investigations and interventions in cancer.
An example of such a target is the F19 cell surface glycoprotein, which is expressed in the reactive stroma fibroblasts of more than 90% of common epithelial cancers, including carcinomas of breast, colon, lung, bladder and pancreas, with little or no expression in normal adult tissues. The F19 cell surface glycoprotein and various teachings regarding it are found in Garin-Chesa et al., Proc. Natl. Acad. USA 87: 7235-7239 (1990); Rettig et al., Proc. Natl. Acad. Sci. USA 85: 3110-3114 (1988); and U.S. Pat. No. 5,059,523, all three disclosures hereby being incorporated by reference. In a recent, phase I study, it has been found that .sup.131 I labeled monoclonal antibody against F19 accumulates at tumor sites, thereby allowing tumor imaging in patients with hepatic metastases from colorectal carcinomas. See Welt et al., Proc. Am. Assoc. Cancer Res. 33: 319 (1992) regarding this imaging study.
Immunologic targeting of tumor vascular endothelial cells has not yet been accomplished, but is attractive for several reasons. One reason is that endothelial surface antigens are highly accessible to antibodies or antibody conjugates which circulate in the blood. Another reason is that the destruction or impairment of blood vessels associated with tumors would be expected to lead to widespread necrosis or arrest of growth of solid tumors. The activity of several antitumor agents, including tumor necrosis factor (TNF-.alpha.), gamma interferon (IFN-.delta.), and melphalan may result from vascular endothelial cell damage rather than direct tumor killing. See Old, Science 230: 630-632 (1985); Lienard et al., J. Clin. Oncol. 10: 52-60 (1992); Lejeune, Eur. Cytok. Net. 2: 124 (1992) for information on these studies.
The targeting of tumor vascular endothelial cells, discussed supra, requires the availability of a monoclonal antibody ("mAb") which is specific for these cells. While the field of immunology as it relates to production of monoclonal antibodies has made great strides since 1975 when Kohler & Milstein first succeeded in generating hybridomas, preparation of monoclonal antibodies with a desired cell type specificity is hardly simple or routine. For example, one must assume that an antigen of requisite specificity exists, or is expressed on the targeted cell, and this is not necessarily the case. This is essential for specificity in general, and is critical for vascular tissue, because any mAb which binds to vascular tissue generally rather than to tumor vascular endothelial cells specifically, will target normal vascular tissues, leading to obvious adverse consequences.
While mAbs to endothelial cells and to tumors originating therefrom are known, the art has not previously been aware of monoclonal antibodies which are specific to tumor vascular endothelium to the exclusion of other non-transformed cell types. Such monoclonal antibodies have, however, now been prepared, and the cell surface antigen to which they are directed has been identified, isolated, and characterized. These, as well as the ramifications thereof, are the subject of the disclosure which follows.